printk: Hide console waiter logic into helpers (c162d5b4) · Commits · 戴 / test

kernel/printk/printk.c

+148 −97

Original line number	Diff line number	Diff line
		@@ -86,15 +86,8 @@ EXPORT_SYMBOL_GPL(console_drivers);
		static struct lockdep_map console_lock_dep_map = {
		.name = "console_lock"
		};
		static struct lockdep_map console_owner_dep_map = {
		.name = "console_owner"
		};
		#endif

		static DEFINE_RAW_SPINLOCK(console_owner_lock);
		static struct task_struct *console_owner;
		static bool console_waiter;

		enum devkmsg_log_bits {
		__DEVKMSG_LOG_BIT_ON = 0,
		__DEVKMSG_LOG_BIT_OFF,
		@@ -1550,6 +1543,146 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
		return do_syslog(type, buf, len, SYSLOG_FROM_READER);
		}

		/*
		* Special console_lock variants that help to reduce the risk of soft-lockups.
		* They allow to pass console_lock to another printk() call using a busy wait.
		*/

		#ifdef CONFIG_LOCKDEP
		static struct lockdep_map console_owner_dep_map = {
		.name = "console_owner"
		};
		#endif

		static DEFINE_RAW_SPINLOCK(console_owner_lock);
		static struct task_struct *console_owner;
		static bool console_waiter;

		/**
		* console_lock_spinning_enable - mark beginning of code where another
		* thread might safely busy wait
		*
		* This basically converts console_lock into a spinlock. This marks
		* the section where the console_lock owner can not sleep, because
		* there may be a waiter spinning (like a spinlock). Also it must be
		* ready to hand over the lock at the end of the section.
		*/
		static void console_lock_spinning_enable(void)
		{
		raw_spin_lock(&console_owner_lock);
		console_owner = current;
		raw_spin_unlock(&console_owner_lock);

		/* The waiter may spin on us after setting console_owner */
		spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
		}

		/**
		* console_lock_spinning_disable_and_check - mark end of code where another
		* thread was able to busy wait and check if there is a waiter
		*
		* This is called at the end of the section where spinning is allowed.
		* It has two functions. First, it is a signal that it is no longer
		* safe to start busy waiting for the lock. Second, it checks if
		* there is a busy waiter and passes the lock rights to her.
		*
		* Important: Callers lose the lock if there was a busy waiter.
		* They must not touch items synchronized by console_lock
		* in this case.
		*
		* Return: 1 if the lock rights were passed, 0 otherwise.
		*/
		static int console_lock_spinning_disable_and_check(void)
		{
		int waiter;

		raw_spin_lock(&console_owner_lock);
		waiter = READ_ONCE(console_waiter);
		console_owner = NULL;
		raw_spin_unlock(&console_owner_lock);

		if (!waiter) {
		spin_release(&console_owner_dep_map, 1, _THIS_IP_);
		return 0;
		}

		/* The waiter is now free to continue */
		WRITE_ONCE(console_waiter, false);

		spin_release(&console_owner_dep_map, 1, _THIS_IP_);

		/*
		* Hand off console_lock to waiter. The waiter will perform
		* the up(). After this, the waiter is the console_lock owner.
		*/
		mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
		return 1;
		}

		/**
		* console_trylock_spinning - try to get console_lock by busy waiting
		*
		* This allows to busy wait for the console_lock when the current
		* owner is running in specially marked sections. It means that
		* the current owner is running and cannot reschedule until it
		* is ready to lose the lock.
		*
		* Return: 1 if we got the lock, 0 othrewise
		*/
		static int console_trylock_spinning(void)
		{
		struct task_struct *owner = NULL;
		bool waiter;
		bool spin = false;
		unsigned long flags;

		if (console_trylock())
		return 1;

		printk_safe_enter_irqsave(flags);

		raw_spin_lock(&console_owner_lock);
		owner = READ_ONCE(console_owner);
		waiter = READ_ONCE(console_waiter);
		if (!waiter && owner && owner != current) {
		WRITE_ONCE(console_waiter, true);
		spin = true;
		}
		raw_spin_unlock(&console_owner_lock);

		/*
		* If there is an active printk() writing to the
		* consoles, instead of having it write our data too,
		* see if we can offload that load from the active
		* printer, and do some printing ourselves.
		* Go into a spin only if there isn't already a waiter
		* spinning, and there is an active printer, and
		* that active printer isn't us (recursive printk?).
		*/
		if (!spin) {
		printk_safe_exit_irqrestore(flags);
		return 0;
		}

		/* We spin waiting for the owner to release us */
		spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
		/* Owner will clear console_waiter on hand off */
		while (READ_ONCE(console_waiter))
		cpu_relax();
		spin_release(&console_owner_dep_map, 1, _THIS_IP_);

		printk_safe_exit_irqrestore(flags);
		/*
		* The owner passed the console lock to us.
		* Since we did not spin on console lock, annotate
		* this as a trylock. Otherwise lockdep will
		* complain.
		*/
		mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);

		return 1;
		}

		/*
		* Call the console drivers, asking them to write out
		* log_buf[start] to log_buf[end - 1].
		@@ -1760,56 +1893,8 @@ asmlinkage int vprintk_emit(int facility, int level,
		* semaphore. The release will print out buffers and wake up
		* /dev/kmsg and syslog() users.
		*/
		if (console_trylock()) {
		console_unlock();
		} else {
		struct task_struct *owner = NULL;
		bool waiter;
		bool spin = false;

		printk_safe_enter_irqsave(flags);

		raw_spin_lock(&console_owner_lock);
		owner = READ_ONCE(console_owner);
		waiter = READ_ONCE(console_waiter);
		if (!waiter && owner && owner != current) {
		WRITE_ONCE(console_waiter, true);
		spin = true;
		}
		raw_spin_unlock(&console_owner_lock);

		/*
		* If there is an active printk() writing to the
		* consoles, instead of having it write our data too,
		* see if we can offload that load from the active
		* printer, and do some printing ourselves.
		* Go into a spin only if there isn't already a waiter
		* spinning, and there is an active printer, and
		* that active printer isn't us (recursive printk?).
		*/
		if (spin) {
		/* We spin waiting for the owner to release us */
		spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
		/* Owner will clear console_waiter on hand off */
		while (READ_ONCE(console_waiter))
		cpu_relax();

		spin_release(&console_owner_dep_map, 1, _THIS_IP_);
		printk_safe_exit_irqrestore(flags);

		/*
		* The owner passed the console lock to us.
		* Since we did not spin on console lock, annotate
		* this as a trylock. Otherwise lockdep will
		* complain.
		*/
		mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
		if (console_trylock_spinning())
		console_unlock();
		printk_safe_enter_irqsave(flags);
		}
		printk_safe_exit_irqrestore(flags);

		}
		}

		return printed_len;
		@@ -1910,6 +1995,8 @@ static ssize_t msg_print_ext_header(char *buf, size_t size,
		static ssize_t msg_print_ext_body(char *buf, size_t size,
		char *dict, size_t dict_len,
		char *text, size_t text_len) { return 0; }
		static void console_lock_spinning_enable(void) { }
		static int console_lock_spinning_disable_and_check(void) { return 0; }
		static void call_console_drivers(const char *ext_text, size_t ext_len,
		const char *text, size_t len) {}
		static size_t msg_print_text(const struct printk_log *msg,
		@@ -2196,7 +2283,6 @@ void console_unlock(void)
		static u64 seen_seq;
		unsigned long flags;
		bool wake_klogd = false;
		bool waiter = false;
		bool do_cond_resched, retry;

		if (console_suspended) {
		@@ -2291,31 +2377,16 @@ skip:
		* finish. This task can not be preempted if there is a
		* waiter waiting to take over.
		*/
		raw_spin_lock(&console_owner_lock);
		console_owner = current;
		raw_spin_unlock(&console_owner_lock);

		/* The waiter may spin on us after setting console_owner */
		spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
		console_lock_spinning_enable();

		stop_critical_timings(); /* don't trace print latency */
		call_console_drivers(ext_text, ext_len, text, len);
		start_critical_timings();

		raw_spin_lock(&console_owner_lock);
		waiter = READ_ONCE(console_waiter);
		console_owner = NULL;
		raw_spin_unlock(&console_owner_lock);

		/*
		* If there is a waiter waiting for us, then pass the
		* rest of the work load over to that waiter.
		*/
		if (waiter)
		break;

		/* There was no waiter, and nothing will spin on us here */
		spin_release(&console_owner_dep_map, 1, _THIS_IP_);
		if (console_lock_spinning_disable_and_check()) {
		printk_safe_exit_irqrestore(flags);
		return;
		}

		printk_safe_exit_irqrestore(flags);

		@@ -2323,26 +2394,6 @@ skip:
		cond_resched();
		}

		/*
		* If there is an active waiter waiting on the console_lock.
		* Pass off the printing to the waiter, and the waiter
		* will continue printing on its CPU, and when all writing
		* has finished, the last printer will wake up klogd.
		*/
		if (waiter) {
		WRITE_ONCE(console_waiter, false);
		/* The waiter is now free to continue */
		spin_release(&console_owner_dep_map, 1, _THIS_IP_);
		/*
		* Hand off console_lock to waiter. The waiter will perform
		* the up(). After this, the waiter is the console_lock owner.
		*/
		mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
		printk_safe_exit_irqrestore(flags);
		/* Note, if waiter is set, logbuf_lock is not held */
		return;
		}

		console_locked = 0;

		/* Release the exclusive_console once it is used */

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